The present invention relates generally to a system for determining the inclination of a bottom-hole assembly as part of a measurement while drilling ("MWD") system or a logging while drilling ("LWD") system. More particularly, the present invention relates to a system for determining the inclination of the hole at the drill bit to permit more precise control of the direction in which the bottom-hole assembly is drilled. Still more particularly, the present invention relates to an MWD system that indirectly determines the inclination of the hole at the drill bit by using measurements from two separate inclinometer sensors positioned at discrete locations in the bottom-hole assembly.
Modern petroleum drilling and production operations demand a great quantity of information relating to parameters and conditions downhole. Such information typically includes the location and orientation of the wellbore and drilling assembly, earth formation properties, and drilling environmental parameters downhole. Directional information relating to surveying the location of the wellbore, and controlling or "steering" the drilling assembly, will be discussed later.
The collection of information relating to formation properties and conditions downhole, commonly referred to as "logging," can be performed by several methods. Oil well logging has been known in the industry for many years as a technique for providing information to a driller regarding the particular earth formation being drilled. In conventional oil well wireline logging, a probe or "sonde" housing formation sensors is lowered into the borehole after some or all of the well has been drilled, and is used to determine certain characteristics of the formations traversed by the borehole. The sonde is supported by a conductive wireline, which attaches to the sonde at the upper end. Power is transmitted to the sensors and instrumentation in the sonde through the conductive wireline. Similarly, the instrumentation in the sonde communicates information to the surface by electrical signals transmitted through the wireline.
More recently, those in the industry have placed an increased emphasis on the collection of data during the drilling process itself. By collecting and processing data during the drilling process, without the necessity of removing or tripping the drilling assembly to insert a wireline logging tool, the driller can make accurate modifications or corrections on-the-fly, as necessary, to optimize performance. Designs for measuring conditions downhole and the movement and location of the drilling assembly, contemporaneously with the drilling of the well, have come to be known as "measurement-while-drilling" techniques, or "MWD." Similar techniques, concentrating more on the measurement of formation parameters, commonly have been referred to as "logging while drilling" techniques, or "LWD." While distinctions between MWD and LWD may exist, the terms MWD and LWD often are used interchangeably. For the purposes of this disclosure, the term MWD will be used with the understanding that this term encompasses both the collection of formation parameters and the collection of information relating to the movement and position of the drilling assembly.
The most common type of MWD sensor is a directional or directional orientation (synonymous terms) sensor. Directional MWD sensors typically comprise a three axis accelerometer and a three axis magnetometer, housed together in the same directional orientation sensor. See "State of the Art in MWD," International MWD Society (January 1993). Normally, drill string tubing is made of steel, which is a magnetic material. According to normal practice, and to prevent interference with the magnetometer readings, the sub housing the directional sensors typically comprises a length of tubing made of a non-magnetic material. Normally, the sub containing the directional sensors is positioned a relatively large distance above the drill bit and/or motor. For example, when a motor is used in the bottomhole assembly, the directional sensor typically is located approximately 50 feet or more above the drill bit.
In accordance with known techniques, wellbore directional measurements can be made as follows: a three axis accelerometer measures the earth's gravitational field vector, relative to the tool axis and a point along the circumference of the tool called the tool face scribe line. From this measurement, the inclination of the bottomhole assembly can be determined to provide an indication of the deviation of the wellbore with respect to vertical. The three axis accelerometer also provides a measure of "tool face angle," which is the orientation (rotational about the tool axis) angle between the scribe line and the high side of the wellbore. Additionally, a three axis magnetometer measures the earth's magnetic field vector in a similar manner. From the combined magnetometer and accelerometer data, the azimuth and magnetic tool face angle of the tool may be determined. As one skilled in the art will understand, azimuth is the horizontally projected direction of the wellbore relative to North.
Because the directional sensors typically are mounted a substantial distance above the bit, the inclination measured by the three axis accelerometer in the directional sub may or may not be a fair representation of the inclination at the bit. In certain directional or horizontal wells, the build rate of the well may exceed 10.degree. per 100 feet. Thus, the inclination measured at the directional sensor may not accurately reflect the inclination at the bit. Inclination is an extremely important parameter for the driller to consider when making decisions regarding course changes and corrections, and the like. As a result, there has been a desire in the drilling industry to obtain inclination information nearer the hole bottom (i.e., at the drill bit) in more nearly "real-time." Placing the directional sensor closer to the bottom of the drill string is risky, however, because of the potential problem of magnetic interference to the azimuth sensors caused by components near the bit, such as the downhole motor. Moreover, the use of a downhole motor makes it difficult to recover sensed values from sensors positioned below the motor because of the practical problems related to placement of, and communication with, the sensors. One attempt to obtain more accurate drilling information at or near the bit is found in U.S. Pat. No. 5,160,925, the teachings of which are incorporated by reference herein. In that patent, a short hop electromagnetic link is used to permit data sensed below the motor to be transmitted around the motor real-time. While this invention provides a viable solution to the problem, such a system is both complicated and expensive. It would therefore be desirable if an alternative system could be developed to provide information reflecting inclination at the bit which is less expensive and simpler to obtain. Although the advantages of such a system are apparent, to date no such system has been developed except for the solution obtained in U.S. Pat. No. 5,160,925.